The present invention relates to mechanical drives of the type commonly referred to as "strap" or tape drives.
Tape drives have been employed heretofore in various mechanical devices. Such drives typically incorporate a drum and a flexible but substantially inextensible, thin, metallic tape having one end fixed to the drum. The opposite end of the tape is fixed to another element arranged for reciprocating or pivoting motion. As the drum pivots about a drum axis, the tape is wound onto the circumferential surface of the drum, and the tape pulls the other element. Because one end of the tape is positively mounted to the drum and the opposite end of the tape is positively mounted to the other member, these devices do not depend upon frictional interengagement between the tape and the drum. Accordingly, devices of this nature are capable of providing a precise, repeatable linkage between the drum and another mechanical element. As disclosed, for example, in U.S. Pat. No. 3,614,898, this other element may be arranged for translational movement, and the system may be arranged to that the drum is the driven element rather than the driving element. Also, two tapes, extending in generally opposite directions from the drum to the other element can be employed to provide positive linkage in both directions of movement. Generally similar tape drives are disclosed in U.S. Pat. Nos. 4,419,707 and 3,267,812.
Tape drives offer considerable advantages of simplicity, precision, silent operation and the like. However, the load carrying capacity of tape drives heretofore has been limited by certain apparently inherent design considerations. As the metallic tape must flex as it winds onto the drum surface, the tape must be thin to minimize bending stress. The single tape thickness typically cannot be increased to provide increased load carrying capacity without also increasing the radius of the drum and hence increasing the size of the device. The most typical approach to this problem heretofore has been to place multiple thin tapes side-by-side on a drum, as shown for example in U.S. Pat. No. 1,405,852, or to provide auxiliary driving elements, such as frictionally engaged elements, as shown in the aforementioned U.S. Pat. No. 3,267,812. Neither of these provides a truly satisfactory solution, inasmuch as frictional elements are subject to slippage and wear, and multiple tapes disposed in a side-by-side arrangement require a wider drum, thereby also increasing the size of the device.
As illustrated in U.S. Pat. No. 2,916,922, attempts have been made heretofore to solve these problems by using a doubled tapes, having two superposed plies. Thus, in the '922 patent, a single elongated strap is folded double to make two plies, engaged with a rotary drum and fastened thereto, adjusted to have equal tension in both plies and then firmly clamped both to the rotary drum and to a slidable element. Because each individual ply is relatively thin, bending loads are substantially alleviated, and the device therefore can be relatively compact.
However, in a device as illustrated in the '922 patent, only one ply of the tape performs any useful function, and the remaining ply is essentially useless. The two plies of U.S. Pat. No. 2,916,922 tape can be under equal tension only at one point in the range of motion in the device. At all other points in the range of motion, substantially all of the load will be carried by only one of the plies. This is so because, in a device as shown in U.S. Pat. No. 2,916,922 patent, unequal lengths of the two plies are wound onto and off of the drum as the drum pivots. The outer tape, remote from the drum surface is effectively wound onto the outer surface of the inner ply and hence is effectively wound onto a larger drum than the inner ply. For example, where both plies of a tape are fastened at one point to a drum of radius R, and each ply has an individual ply thickness T, the inner ply resting directly on the drum surface will be pulled through a distance equal to (A).times.(R) upon pivoting of the drum through an angle of A radians, whereas the outer ply will be pulled through a distance equal to (A).times.(R+T) during the same pivoting motion. Accordingly, the relative degree of tension in the plies changes as the drum pivots. Even if the two plies are initially adjusted to equal tension at one point in the range of motion, one of the two plies will be substantially slack at every other point.
The problem of unequal tension in the plies of a multi-ply tape is discussed in U.S. Pat. No. 1,982,394. U.S. Pat. No. 1,982,394 employs a multi-ply tape as a guide or motion-transmitting element in an internal combustion engine. In one arrangement (FIGS. 36 and 37 of U.S. Pat. No. 1,982,394 patent) the plies of a multi-ply tape extend around a drum and from the drum to a free end. At the free end, each ply is connected to a separate slidable block, and the slidable blocks in turn are linked to a machine element by a system of further tapes and pivoting elements. Longitudinal forces or tension in the tape can be transmitted through the blocks, to the frame of the machine, but differences in tension of the individual plies can be equalized by sliding movement of the blocks. This arrangement requires a great multiplicity of complex and costly parts, and requires individual fastening of each ply to the corresponding slidable block. Another arrangement disclosed in U.S. Pat. No. 1,982,394 patent (FIG. 7A) shows a multi-ply tape having separate spacer blocks affixed between each pair of plies adjacent to the free end of the tape, and having all of the plies fastened to a common element, which element is free to rock with respect to the frame of the machine. This rockable element is in turn provided with cam-shaped or "lenticular" surfaces to engage the plies of the tape, so that pivoting movement of the rockable element will assertedly equalize the tension among the various plies of the tape. This arrangement suffers from similar disadvantages of cost, complexity and bulk.
Yet another arrangement is shown in FIGS. 1 and 2 of U.S. Pat. No. 1,982,394. Here, the elongated tape itself is constituted by a plurality of straps. Each strap is folded on itself so as to define a bight at the free end of the tape remote from the winding drum. These bights are disposed one inside the other, and extend over a plurality of securement elements, such that each bight rests upon a separate securement element. The securement elements include a central post and a plurality of C-shaped shells concentric with the central post but spaced therefrom. The innermost bight rests upon the central post, whereas the outer bight rests upon individual C-shaped shells. The plies of the tape thus extend from the bights at the free end into two separate runs, one passing to each side of the post. These separate runs merge with one another at the point were the tape wraps onto the drum. The individual plies in each run are spaced apart from one another adjacent the bights, at the free end of the tape, but contiguous with one another where they wrap onto the drum. Accordingly, the individual plies in each run are not parallel with one another. The securement elements are arranged to rock as the drive operates, assertedly to equalize the tension in the various plies of the tape. This arrangement still requires considerable complexity and costs. Moreover, as further explained hereinbelow, this arrangement does not provide exact compensation for the unequal motions of the various plies in the tape, and hence does not maintain the optimum, equal load distribution among the various plies throughout the range of motion of the device.
Additionally, attempts to provide multi-ply tape drives have encountered problems in securing the ends of the individual plies to the machine elements. Thus, U.S. Pat. No. 2,916,922 patent employs a clamp squeezing both plies of the two-ply tape against the drum. U.S. Pat. No. 1,982,394 patent utilizes in some embodiments a clamp or key way bearing on all of the plies and in other embodiments individual securements of the ends of the plies. Attempts to clamp all of the plies together so as to hold the one end of the tape to one of the machine elements work only with a limited number of plies. Where the entire tape is clamped, the friction between the clamping elements and the immediately adjacent plies of the tape must be great enough to withstand the entire load resulting from the tension on all of the plies. The clamping forces exerted by the clamping elements normal to the faces of the tape must be correspondingly large. These forces induce a significant stress in the plies, particularly where there are a great number of very thin plies. The other arrangement, in which each ply is individually clamped to the machine element or drum introduces unacceptable bulk, complexity and cost where there are a great number of plies.
Accordingly, there have been significant, unmet needs for improvements in tape drives and linkages.